The main antagonist could be an ancient snake spirit that was bound to the contract. This spirit might not like the idea of Naruto, a human, having control over the powers of the contract. So it tries to possess Naruto or drive him insane by constantly whispering dark thoughts and trying to make him use the snake powers for evil. It would be a battle of wills between Naruto and this malevolent spirit.
The plot could be that Naruto signs the snake contract in a moment of desperation. For example, during a battle where the village is on the verge of destruction and all other options seem exhausted. After signing, he discovers that the snake contract gives him access to a hidden dimension filled with snake - like creatures and ancient knowledge. He has to train in this dimension to master the powers of the contract. But there are also other forces in this dimension that want to use Naruto for their own evil purposes. He then has to fight against these internal threats while also using his new - found snake powers to defend his village from external enemies.
Maybe it's an ancient evil that was sealed away long ago and the colossus contract is the key to its release. This evil force would be the main antagonist as it tries to break free and cause chaos in the ninja world. It could be something like a powerful demon that was once defeated by the first Hokage but is now on the verge of coming back because of the colossus contract. Naruto would have to stop it with the power of the colossus.
Naruto himself could be an interesting main character in a snake contract fanfiction. Imagine Naruto having to take on the snake contract for some reason, like to save Sasuke who has fallen deeper into Orochimaru's influence. It would be a huge change for him as he's always been associated with the toad contract. But with his determination, he could learn to master the snake powers in his own unique way.
It could be someone who is jealous of Naruto getting the monkey contract. Maybe a rogue ninja who wants the power for himself.
Well, it could mean he gains new powers related to the monkeys. Maybe he can summon really powerful monkey-like creatures to help him in battles. For example, he might be able to call on monkeys with unique jutsu or abilities that normal summons don't have.
The dogs themselves would be very important characters. There could be a lead dog, like a wise and powerful alpha dog that Naruto forms a close connection with. Maybe Akamaru could also play a role, since he is a dog in the Naruto universe already and could be involved in some way with Naruto's new contract.
Well, Naruto might gain some unique abilities from the dog contract. For example, he could have enhanced senses like better smell or hearing. Maybe he'd be able to communicate with dogs more effectively and use them in battles in new and creative ways.
A rogue ninja who wants to steal the power of the contract for his own evil purposes could be the main antagonist. He might be jealous of the connection between Sakura and Itachi and wants to break it to gain the abilities they could potentially unlock together.
There could be a jealous student. This student might be envious of Harry getting all the attention with his snake - related adventures. So, they would act as an antagonist by trying to undermine Harry's efforts in the snake fanfiction world. They could spread false rumors about snakes being dangerous or Harry's intentions being dark, causing trouble for Harry among his peers and teachers.
Well, in this fanfiction, Naruto might gain some unique powers. Maybe he could communicate with the fox in a special way, like having telepathic conversations. This could lead to him having an edge in battles as he could quickly call upon the fox's strength.